1. The Field of the Invention
This application relates to optical transmitters and, more particularly to systems and methods for overcoming chromatic dispersion.
2. The Relevant Technology
The ability to transmit data over optical fibers over long distances is limited by various factors. For example, at intermediate stages between transmitter and receiver a signal may be amplified, which introduces noise into the signal. The signal may also be converted to an electrical signal and then retransmitted, which is subject to detection errors.
The primary limitation on long-haul transmission of optical signals is dispersion within the optical fiber itself Inasmuch as the optical fiber has a wavelength dependent index of refraction, different frequency components of a signal travel at different speeds. Transmitted pulses will therefore tend to broaden, causing the peak amplitude of 1 bits to be reduced and the amplitude of adjacent 0 bits to increase thereby making the transmitted symbols indistinguishable from one another.
U.S. patent application Ser. No. 11/084,633, filed Mar. 18, 2005, and entitled “Method and apparatus for transmitting a signal using simultaneous FM and AM modulation,” described a laser transmitter including a directly modulated laser that emits frequency modulated pulses through an optical spectrum reshaper that converts a portion of the frequency modulation to amplitude modulation. In this application, the frequency modulated signal includes frequency excursions from a base frequency to a peak frequency. The application discloses that for frequency excursions equal to between 0.25 and 0.75 times a bit rate, 1 bits separated by an odd number of 0 bits will destructively interfere as they broaden and begin to overlap, which makes an intervening 0 bit more readily distinguishable. Although this method is particularly useful for promoting proper detection of the 101 bit sequence, it does not provide any benefit for isolated 1 bits among multiple 0 bits.
In view of the foregoing, it would be an advancement in the art to provide a laser transmitter having a directly modulated laser and optical spectrum reshaper that reduces errors caused by chromatic dispersion for a plurality of bit patterns, particularly isolated 1 bits.